There is a highly specific temporal ordering of rhythmic patterns in physiological systems. Sleep-wake cycles, growth-hormone secretion, potassium secretion, and cortisol levels all fluctuate in a precise relationship with the circadian temperature rhythm (CTR). The timing of these physiological processes is quite similar across numerous species. There is a growing body of evidence that loss of temporal organization among various biological rhythm systems is a concomitant of some forms of mental illness such as depression, and manic depressive illness. The mechanisms that link rhythm abberations to mood disorders are unknown, but the data currently being gathered suggest that the association of the two is more than casual. The CTR is one of the rhythm systems that appears disturbed in mood disorders. We have been engaged in studying the ontogeny of the CTR in rats and have reported that the CTR emerges around 25 days of age and continues to develop until 35-40 days. We plan to continue this work by examining behavioral, physiological, and neurophysiological mechanisms that may underlie rhythm ontogeny in the rat with a particular emphasis on the development and subsequent maintenance of the CTR. We measure temperature, drinking, and activity via telemetry signals every 5 min from 10 days of age. Studies on the ontogeny of various rhythms in rodents suggest that both prenatal and postnatal factors affect the development of normal rhythmicity. Our knowledge of rhythm dysfunction effects upon mental health disorders can be enhanced by a better understanding of the ontogenetical processes that produce temporal organization in biological organisms. Light shed on CTR development may reflect on rhythm ontogeny in general and illuminate maturational processes and environmental influences critical to normal functioning.